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Settlement response of embankment on multi layer geosynthetic-reinforced reclaimed ground.

The paper presents a study on the settlement response of embankment on multi-layered geosynthetic reinforced granular fill over reclaimed soil. The granular fill and the soil have been idealized by the Pasternak shear layer and a layer of Winkler springs, respectively. Stretched rough elastic membranes represent the geosynthetic reinforcement layers. Plane strain conditions are considered for the loading and reinforced foundation soil system. An iterative finite difference scheme is applied for obtaining the solution and results are presented in nondimensional form. Significant reduction in the settlement is observed when the number of reinforcement layer is increased.

INTRODUCTION

Coastal lowlands are very soft and highly compressible soils and get inundated by the fluctuating water levels. Reclamation of these lowlands requires raising the ground to a level and makes it safe from the effects of tides and inundation. Reclamation and soil improvement works in these areas are very challenging. To develop these grounds, they have to be reclaimed by placing suitable material over soft ground to improve bearing capacity and to reduce settlements of the structures constructed on them. The embankments are required for highways and railways and are one of the most critical structures to be constructed. The common form of reclamation is to spread and compact granular fill over the soft sub-grade soil. However, this conventional reclamation approach requires huge amount of fill, as fill tends to displace and sinks non-uniformly into the soft soil. To reduce the spreading of the granular material, geosynthetic reinforcement at the base of the granular fill can be used to make the process of reclamation more effective. The use of geosynthetic reinforcements reduces the settlement of the reclaimed soft ground under loading, distributes the loads uniformly and prevents non-uniform sinking of the granular material into the ground. The behavior of the reclaimed ground can be further improved by providing single or multi layer of reinforcements within the granular fill. In recent years, many researchers have studied load-settlement behavior of such reinforced foundation beds based on lumped parameter modeling (Madhav and Poorooshasb, 1988; Ghosh and Madhav, 1994; Shukla and Chandra, 1995; Yin, 1997; Ramu, 2001; Maheshwari et al., 2004).

However, most of the models reported in the literature are developed for single layer reinforced systems. In this paper, the settlement response of embankment on multilayered geosynthetic reinforced granular fill over reclaimed soft soil has been studied. Parametric studies have been carried out to find the load-settlement behavior of the multi layer geosynthetic-reinforced soil as well as that of the unreinforced soil.

STATEMENT OF THE PROBLEM

A multi layer geosynthetic-reinforced granular fill on reclaimed soft foundation soil is shown in Figure 1. The proposed foundation model as shown in Figure 2 may idealize the behavior of such a system. The stress-strain behavior of the granular fill and the soft soil is considered as linear. Three geosynthetic layers are considered in the model and they divide the shear layer into four equal parts. The shear modulus of the granular fill is taken as constant throughout the study. It is assumed that the geosynthetic reinforcement layers are rough enough to prevent slippage at the interface with soil.

[FIGURES 1-2 OMITTED]

An embankment loading is applied over a width of 2B on the multi layer geosynthetic-reinforced granular fill of width 2L over soft soil as shown in Figure 2. The intensity of loading depends on the height of the embankment and density of the embankment soil. Higher height will be more loading intensity. Additional surcharge loading may also act on the embankment as traffic load. The analysis of the problem so defined is carried out by extending the single layer reinforcement model reported by Shukla and Chandra (1995) for multi-layered reinforced system.

Following the procedures described by Shukla and Chandra (1995) and using the nondimensional parameters as: X=x/B; W=w/B; [G.sup.*]=GH/[k.sub.s][B.sup.2]; [T.sub.j.sup.*]=[T.sub.j]/[k.sub.s][B.sup.2]; [T.sub.p]*=[T.sub.p]/[k.sub.s][B.sup.2] and [q.sup.*]=q/ksB, one gets the following governing equations for multi-layered reinforced system,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)

and H and G are the thickness and the shear modulus of the granular layers, respectively; w is the vertical displacement; q is the loading intensity; [T.sub.1], [T.sub.2] and [T.sub.3] are the tension mobilized in the top, middle and bottom geosynthetic layer, respectively; [T.sub.p] is the pretension force applied to the geosynthetic layers; [mu] is the interface friction at the top and bottom of the geosynthetic layers; e is the slope of the membrane and tan[theta] = -dw/dx; [K.sub.0] is the coefficient of lateral earth pressure at rest is assumed to be equal to 1--sin[phi] (Brooker and Ireland, 1965); and [k.sub.s] is the modulus of the subgrade reaction (spring constant) for the Winkler spring layer as shown in Figure 2.

METHOD OF SOLUTION

Finite difference formulation Finite difference method has been employed to solve the differential equations [Equations (1)-(4)]. In these equations, the derivative [d.sup.2]W/d[X.sup.2] has been expressed by central difference scheme while d[T.sub.j.sup.*]/dX have been expressed by forward difference scheme. The length L/B may be divided into n number of the same increment length with (n+1) number of node points (i = 1, 2, 3, 4.........., n).

Boundary conditions and loading

Due to symmetry of the system only half portion is taken into consideration. At X = 0, due to symmetry, the slope, dW/dX, will be zero and at X = L/B, the geosynthetic layers are free, so that the mobilized tension in different geosynthetic layers are zero, i.e. [T.sub.j.sup.*]= 0. The shear stress acting on the shear layer at the edge (at X = L/B) will also be zero since there is no confinement, i.e. dW/dX = 0. The loading conditions that are considered are given as: [q.sub.i.sup.*](X) =[q.sup.*] for |X|[less than or equal to]0.5; [q.sub.i.sup.*](X) =2[q.sup.*](1- |X|) for 0.5<|X|[less than or equal to] 1.0.and [q.sub.i.sup.*](X) =0 for |X|>1.0.

RESULTS AND DISCUSSIONS

A computer program based on the formulation as described above has been developed and solutions are obtained using an iterative technique with a tolerance value of 10-4. The typical values used for this study are the angle of shearing resistance for the granular fill, [phi] = 36[degrees]; the coefficient of lateral stress, [K.sub.0] = 0.41; the interface friction coefficients, [mu] = 0.5; [G.sup.*]=0.05; [T.sub.p.sup.*]=0; and L/B = 2.0.

Figure 3 shows the settlement profiles of the embankment for different number of reinforcement layer as well as for unreinforced soil for various loading intensities. For loading intensity 0.3, as the number of geosynthetic layer increases from zero to three, the settlement at the centre of the loaded region is decreased by 19.7%, whereas for loading intensity 0.8, this reduction is 33.1%. Thus, for higher loading intensity multiple geosynthetic reinforcements reduce the vertical settlement more effectively than for lower loading intensity. This is due to the fact that tension mobilization of geosynthetic reinforcements increases with increasing load. It is further observed that at [q.sup.*] = 0.8, the reduction of the settlement at the centre of the loaded region is 18.2%, 26.6% and 33.1% from unreinforced to one, two, and three layer geosynthetic reinforced soil system, respectively. However, at the edge of the loaded region the difference of the settlement for different number of reinforcement layer is less that 1%. The results indicate that application of multiple geosynthetic reinforcements reduce both the total and differential settlements of the loaded region.

[FIGURE 3 OMITTED]

Figure 4 shows the mobilized tension in the reinforcement layers of a three layer reinforced system for a particular loading intensity ([q.sup.*]= 0.8). It can be observed that the top geosynthetic layer is subjected to higher tension up to x/B [less than or equal to] 0.3, whereas beyond that region the mobilized tension in bottom geosynthetic layer is higher. It can be further seen that, beyond a distance equal to the width of the embankment reinforcements are not very effective as there is no significant amount of mobilized tension beyond that length.

[FIGURE 4 OMITTED]

CONCLUSIONS

Application of multiple geosynthetic reinforcements reduces the total as well as the differential settlements of the loaded region of the reclaimed soil and the use of multiple geosynthetic-reinforced systems is more effective at the higher load intensities. As the number of reinforcement layer increases the maximum settlement of the embankment decreases in a decreasing rate. The top reinforcement layer is subjected to higher mobilized tension than the lower layers and it is maximum at the center of the loaded region.

REFERENCES

Brooker, E. W. and Ireland, H. O. (1965). "Earth pressure at rest related to stress history", Canadian Geotechnical Journal, Vol. 2, No. 1-2, 1-15.

Ghosh, C. and Madhav, M. R. (1994). "Settlement response of a reinforced shallow earth bed", Geotextile and Geomembranes, Vol. 13, No. 9, 643-656.

Madhav, M. R. and Poorooshasb, H. B. (1988). "A new model for geosynthetic-reinforced soil", Computers and Geotechnics, Vol. 6, No. 4, 277-290.

Maheshwari, P., Basudhar, P. K. and Chandra, S. (2004). "Analysis of beams on reinforced granular beds", Geosynthetics International, Vol. 11, No. 6, 470-480.

Ramu, K. (2001). "Modeling approaches for & analysis of reclamation process and response of reclaimed ground", PhD thesis, Department of Civil Engineering, IIT Kanpur.

Shukla, S. K. and Chandra, S. (1995). "Modeling of geosynthetic-reinforced engineered granular fill on soft soil", Geosynthetics International, Vol. 2, No. 3, 603-617.

Yin, J. H. (1997). "Modeling geosynthetic-reinforced granular fills over soft soil", Geosynthetics International, Vol. 4, No. 2, 165-185.

KOUSIK DEB

Research Scholar, Department of Civil Engineering, Indian Institute of Technology, Kanpur, Kanpur 208016, India, Email: kdeb@iitk.ac.in

SARVESH CHANDRA

Professor, Department of Civil Engineering, Indian Institute of Technology, Kanpur, Kanpur 208016, India, Telephone: +91-512-259-7667, Telefax: +91-512-259-7395, Email: sarv@iitk.ac.in

P. K. BASUDHAR

Professor, Department of Civil Engineering, Indian Institute of Technology, Kanpur, Kanpur 208016, India, Telephone: +91-512-259-7029, Telefax: +91-512-259-7395, Email: pkbd@iitk.ac.in
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Article Details
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Author:Deb, Kousik; Chandra, Sarvesh; Basudhar, P.K.
Publication:Geotechnical Engineering for Disaster Mitigation and Rehabilitation
Article Type:Conference news
Geographic Code:9INDI
Date:Jan 1, 2005
Words:1749
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